1. Field of the Invention
The present invention relates to a hydraulic recovery apparatus for use in a construction machine such as a hydraulic excavator, and a construction machine using the hydraulic recovery apparatus.
2. Description of the Related Art
For example, a hydraulic excavator usually comprises a lower travel structure; an upper swing structure rotatably mounted on the lower travel structure; a multi-articulated front mechanism rotatably coupled to the upper swing structure and including a boom, an arm and a bucket; and a plurality of actuators including a boom hydraulic cylinder, an arm hydraulic cylinder and a bucket hydraulic cylinder for driving the boom, the arm and the bucket, respectively.
For some of among the plurality of actuators, a higher actuator speed has recently been required, as operators have become skillful in operation of a hydraulic excavator. When performing an arm crowding operation, for example, the arm is preferably operated at a higher speed from the standpoint of work efficiency during a stroke until the bucket reaches the ground surface. In such a case, therefore, associated mechanisms are required to operate at higher speeds.
As a means for meeting that demand for speed-up, there is known a hydraulic recovery apparatus including a recovery circuit which returns a hydraulic fluid on the rod side of a hydraulic cylinder to the bottom side with a selector valve or the like for increasing the speed at which a cylinder rod is extended at the same pump delivery rate, thereby recovering energy (or keeping the same speed at a smaller pump delivery rate). Such a conventional hydraulic recovery apparatus is disclosed in, e.g., JP,A 3-117704.
The disclosed hydraulic recovery apparatus is equipped in a hydraulic drive system for a construction machine in which a plurality of actuators, such as a boom hydraulic cylinder, an arm hydraulic cylinder and a bucket hydraulic cylinder, are driven by a hydraulic fluid supplied from a hydraulic pump that is driven by a prime mover such as an engine. Then, the disclosed hydraulic recovery apparatus comprises a first line for supplying the hydraulic fluid to the bottom side of the arm hydraulic cylinder; a second line for draining the hydraulic fluid from the rod side of the arm hydraulic cylinder; and a hydraulic selector valve including a recovery line for supplying at least a part of the hydraulic fluid from the second line to the first line, and a drain line for returning the remaining part of the hydraulic fluid, which is not recovered, from the second line to a hydraulic reservoir through restricting means.
In that hydraulic recovery apparatus, during the arm crowding operation where the hydraulic selector valve is shifted to one side and the hydraulic fluid is supplied to a bottom side hydraulic chamber of the arm hydraulic cylinder, when the load imposed on the arm hydraulic cylinder is relatively small and the pressure in the bottom side hydraulic chamber is relatively low, most of the hydraulic fluid drained from the rod side of the arm hydraulic cylinder to the second line is introduced to the first line via the recovery line rather than to the drain line in which the restricting means is disposed, and is returned to the bottom side of the arm hydraulic cylinder (joined recovery state). As the load imposed on the arm hydraulic cylinder increases and the pressure in the bottom side hydraulic chamber rises, the amount of the hydraulic fluid introduced to the recovery line is reduced and a larger amount of the hydraulic fluid is introduced to the drain line in which the restricting means is disposed. Finally, the hydraulic fluid is all introduced to only the drain line and then drained to the hydraulic reservoir (end of recovery joining).
In addition, the relationship between the load of the arm hydraulic cylinder and the end of recovery joining can be optionally set by constructing the throttling means as a variable throttle driven with a pilot pressure.
The above-mentioned related art, however, has the following problems.
In the related-art hydraulic recovery apparatus, as described above, the recovery operation is basically performed by simple control, namely, just by switching over the start of recovery joining and the end of recovery joining depending on the load pressure of the arm hydraulic cylinder.
When the operating mode of a hydraulic excavator is changed, for example, from the arm-crowding sole operation to the arm-crowding and bucket-crowding combined operation, a part of the delivery rate from a hydraulic pump is introduced not to the side of the arm hydraulic cylinder, but to the side of the bucket hydraulic cylinder. Even in the case where the load pressure of the arm hydraulic cylinder is relatively low and the system is in the joined recovery state, therefore, the above situation may often result in that the hydraulic fluid cannot be supplied at a sufficient flow rate to the bottom side of the arm hydraulic cylinder in spite of a recovery flow rate being added, and the arm hydraulic cylinder cannot follow the arm crowding operation in a satisfactory manner. Such a deficiency of the supply flow rate causes the occurrence of bubbles (cavitation) in the bottom side hydraulic chamber of the arm hydraulic cylinder and hydraulic circuits connected to it, thus resulting in deterioration of operability and durability.
While the above description is made, by way of example, in connection with a deficiency of the supply flow rate caused upon a shift from the sole operation to the combined operation, the occurrence of a deficiency of the supply flow rate is not limited to such a case. A similar situation also occurs, for example, when the revolution speed of a prime mover for driving the hydraulic pump is reduced, and a similar problem arises in that case as well.
Accordingly, it is an object of the present invention to provide a hydraulic recovery apparatus for a construction machine and a construction machine using the hydraulic recovery apparatus, which can prevent the occurrence of cavitation upon, e.g., a shift to the combined operation and a decrease in revolution speed of a prime mover, and which can improve operability and durability.
(1) To achieve the above object, a hydraulic recovery apparatus for a construction machine, according to the present invention, is provided in a hydraulic drive system for driving a plurality of actuators by a hydraulic fluid supplied from at least one hydraulic pump in the construction machine, and comprises a first line for supplying the hydraulic fluid to the bottom side of at least one particular hydraulic cylinder among the plurality of actuators; a second line for draining the hydraulic fluid from the rod side of the particular hydraulic cylinder; a recovery valve means for supplying at least a part of the hydraulic fluid from the second line to the first line; a second variable throttle provided in the recovery valve means and supplying at least the part of the hydraulic fluid from the second line to the first line at a desired opening; a throttle valve means for returning the remaining part of the hydraulic fluid, which is not recovered, from the second line to a hydraulic reservoir; a first variable throttle provided in the throttle valve means and returning the remaining part of the hydraulic fluid, which is not recovered, to the hydraulic reservoir at a desired opening; and a control means for controlling respective opening areas of the first variable throttle and the second variable throttle depending on an actuator flow rate supplied from the hydraulic pump to the particular hydraulic cylinder.
With the present invention, the second variable throttle is provided in the recovery valve means for supplying a part of the hydraulic fluid from the second line to the first line, and the first variable throttle is provided in the throttle valve means for returning the remaining part of the hydraulic fluid, which is not recovered, from the second line to the hydraulic reservoir. By properly controlling amounts by which the hydraulic fluid is throttled by the second throttle valve and the first throttle valve, therefore, a balance (distribution) between a recovery flow rate recovered from the rod side to the bottom side of the particular hydraulic cylinder and a drain (non-recovery) flow rate not recovered from the rod side to the bottom side of the particular hydraulic cylinder, but drained to the hydraulic reservoir, can be adjusted.
To that end, in the present invention, the control means controls the opening areas of the first variable throttle and the second variable throttle depending on the actuator flow rate supplied from the hydraulic pump to the particular hydraulic cylinder. More specifically, the flow rate of the hydraulic fluid introduced to an arm hydraulic cylinder (i.e., an actuator flow rate supplied to the arm hydraulic cylinder) is often abruptly reduced upon, e.g., a shift of the operating mode of a hydraulic excavator, in which the mode is shifted from the arm-crowding sole operation to the arm-crowding and bucket-crowding combined operation and a part of the delivery rate of the hydraulic pump is introduced to a bucket hydraulic cylinder, or a decrease in revolution speed of a prime mover. In response to such a situation, the opening area of the first variable throttle in the throttle valve means is reduced to decrease the non-recovery flow rate, and the opening area of the second variable throttle in the recovery valve means is increased to increase the recovery flow rate. As a result, the reduction of the actuator flow rate is compensated by increasing the recovery flow rate so that the hydraulic fluid can be continuously supplied at a sufficient flow rate to the bottom side of the arm hydraulic cylinder and the arm hydraulic cylinder can follow the arm crowding operation in a satisfactory manner. It is hence possible to prevent cavitation from occurring in the bottom side hydraulic chamber of the particular hydraulic cylinder (arm hydraulic cylinder in this case) and its peripheral hydraulic circuits due to a deficiency of the supply flow rate, and to improve operability and durability.
(2) In above (1), preferably, the control means comprises an actuator flow rate detecting means for detecting the actuator flow rate, and an opening area varying means for varying the respective opening areas of the first variable throttle and the second variable throttle depending on the detected actuator flow rate.
(3) In above (2), preferably, the actuator flow rate detecting means comprises a delivery rate detecting means for detecting a delivery rate of the hydraulic pump, and a distribution ratio deciding means for deciding a distribution ratio of the detected delivery rate to respective actuators.
(4) In above (3), preferably, the delivery rate detecting means comprises a revolution speed detecting means for detecting a revolution speed of a prime mover for driving the hydraulic pump.
With that feature, even when the revolution speed of the prime mover is changed upon, e.g., an increase in load of any actuator or a shift in setting revolution speed or operating mode of the prime mover, and the delivery rate of the hydraulic pump is changed, the actuator flow rate can be detected with high accuracy responsively. In such a case, therefore, it is also possible to surely prevent cavitation from occurring in the bottom side hydraulic chamber of the particular hydraulic cylinder and peripheral hydraulic circuits connected to it due to a deficiency of the supply flow rate, and to improve operability and durability.
(5) In above (4), preferably, the delivery rate detecting means comprises a plurality of input amount detecting means for detecting respective input amounts of a plurality of operating means for operating the plurality of actuators.
With that feature, even when pump delivery rate control (e.g., negative control, positive control, or load sensing control) is performed depending on the input amounts of the operating means, the actuator flow rate can be detected with high accuracy responsively. In such a case, therefore, it is also possible to surely prevent cavitation from occurring in the bottom side hydraulic chamber of the particular hydraulic cylinder and peripheral hydraulic circuits connected to it due to a deficiency of the supply flow rate, and to improve operability and durability.
(6) Also in above (3), preferably, the distribution ratio deciding means comprises an opening area ratio detecting means for detecting an opening area ratio between a plurality of control valves disposed between the hydraulic pump and the plurality of actuators, respectively, for controlling flows of the hydraulic fluid supplied to the corresponding actuators, and a modifying means for modifying the detected opening area ratio depending on operating states of the plurality of actuators.
(7) Also in above (2), preferably, the opening area varying means comprises first and second throttle flow rate deciding means for deciding respective throttle flow rates through the second variable throttle and the first variable throttle depending on the detected actuator flow rate, and first and second opening area deciding means for deciding respective opening areas of the second variable throttle and the first variable throttle depending on the decided throttle flow rates.
(8) In above (7), preferably, the first throttle flow rate deciding means decides the throttle flow rate through the second variable throttle in accordance with both an inlet setting flow rate at which the hydraulic fluid is introduced to the bottom side of the particular hydraulic cylinder, and the detected actuator flow rate.
(9) In above (8), preferably, the second throttle flow rate deciding means decides the throttle flow rate through the first variable throttle in accordance with the inlet setting flow rate, a volume ratio between a bottom-side hydraulic chamber and a rod-side hydraulic chamber of the particular hydraulic cylinder, and the decided throttle flow rate through the second variable throttle.
(10) Also in above (7), preferably, the first opening area deciding means decides the opening area of the second variable throttle in accordance with the decided throttle flow rate through the second variable throttle, a bottom setting pressure set to prevent the occurrence of cavitation in a bottom-side hydraulic chamber of the particular hydraulic cylinder, a volume ratio between the bottom-side hydraulic chamber and a rod-side hydraulic chamber of the particular hydraulic cylinder, and a holding pressure to be maintained in the particular hydraulic cylinder.
(11) In above (10), preferably, the second opening area deciding means decides the opening area of the first variable throttle in accordance with the decided throttle flow rate through the first variable throttle, the bottom setting pressure, the volume ratio, the holding pressure, and a reservoir pressure in the hydraulic reservoir.
(12) Further, to achieve the above object, a construction machine according to the present invention comprises a lower travel structure; an upper swing structure rotatably mounted on the lower travel structure; a multi-articulated front mechanism rotatably coupled to the upper swing structure and including a boom, an arm and a bucket; a plurality of actuators including a boom hydraulic cylinder, an arm hydraulic cylinder and a bucket hydraulic cylinder for driving the boom, the arm and the bucket, respectively; a first line for supplying a hydraulic fluid to the bottom side of at least one particular hydraulic cylinder among the plurality of actuators; a second line for draining the hydraulic fluid from the rod side of the particular hydraulic cylinder; a recovery valve means for supplying at least a part of the hydraulic fluid from the second line to the first line through a second variable throttle; a throttle valve means for returning the remaining part of the hydraulic fluid, which is not recovered, from the second line to a hydraulic reservoir through a first variable throttle; and a control means for controlling respective opening areas of the first variable throttle and the second variable throttle depending on an actuator flow rate supplied from the hydraulic pump to the particular hydraulic cylinder.
(13) In above (12), preferably, the control means comprises an actuator flow rate detecting means for detecting the actuator flow rate, and an opening area varying means for varying the respective opening areas of the first variable throttle and the second variable throttle depending on the detected actuator flow rate.
(14) In above (12) or (13), preferably, the recovery valve means is disposed, with respect to a particular control valve for controlling a flow of the hydraulic fluid supplied to the particular hydraulic cylinder from the hydraulic pump and to the particular hydraulic cylinder, at a position nearer to at least the particular hydraulic cylinder.
It is a general rule that, when recovering a part of the hydraulic fluid drained from a hydraulic cylinder, the recovery flow rate can be more easily increased as the recovery line pressure on the rod side of the hydraulic cylinder is higher and the recovery line pressure on the bottom side of the hydraulic cylinder is lower. On the other hand, when the hydraulic fluid is supplied to the hydraulic cylinder through a control valve for controlling a flow of the hydraulic fluid from the hydraulic pump, the hydraulic pump, the control valve and the hydraulic cylinder are interconnected in the order named. In that arrangement, if a recovery line is disposed remotely from the hydraulic cylinder, a pressure loss caused in an intermediate line becomes relatively large. Thus, the recovery line pressure on the bottom side is increased because it is positioned closer to the hydraulic pump, and the recovery line pressure on the rod side is reduced by an amount corresponding to the above-mentioned pressure loss. It is hence difficult to obtain a large recovery flow rate.
In view of such a difficulty, in this embodiment, the recovery valve means is disposed at a position nearer to at least the particular hydraulic cylinder of the particular control valve and the particular hydraulic cylinder. With that arrangement, the pressure loss in the recovery line can be reduced so that the pressure at a port of the recovery valve means communicating with the rod side of the particular hydraulic cylinder can be maintained relatively high and the pressure at a port of the recovery valve means communicating with the bottom side thereof can be maintained relatively low. Accordingly, a larger recovery flow rate can be more easily obtained.
(15) In above (14), preferably, the recovery valve means is disposed on the particular hydraulic cylinder.
(16) Also in above (12) or (13), preferably, the recovery valve means is disposed on the boom.
(17) Further in above (12) or (13), preferably, the recovery valve means and the throttle valve means are constructed as an integral unit and are disposed on the boom.